Sucrose is a sugar. The concentration of a solution of sucrose can be measured by the optical rotation, α, of a sucrose solution. The more concentrated the solution, the greater the optical rotation of the solution. A polarimeter is used to measure optical rotation. Light is passed through a sample of the sucrose solution in a glass cell, and the observed angle of rotation, αobs, is measured. A simplified diagram of a polarimeter is shown. [Figure X.X] If a glass cell of length 10 cm is filled with a solution of sucrose of concentration 1 g cm⁻³ the measured angle of rotation is known as the specific rotation, [α]. The observed angle of rotation, αobs, measured by the polarimeter is related mathematically to the concentration of the sucrose solution by the equation shown. αobs = [α]c αobs is the observed angle of rotation using a 10 cm cell [α] is the specific rotation of sucrose solution c is the concentration of sucrose, in g cm⁻³ A student wanted to determine the specific rotation of sucrose, [α]. Solutions of different concentrations of sucrose at 20 °C were placed in a polarimeter and the observed angle of rotation, αobs, recorded. The '+' sign is used to show that the rotation is in a clockwise direction.

1. (a)(i)Plot a graph on the grid on page 9 to show the relationship between concentration of sucrose, c, and observed angle of rotation, αobs. Use a cross (x) to plot each data point. Draw a line of best fit.[2]
2. (a)(ii)Circle the most anomalous point on your graph.[1]
3. (a)(iii)Use the graph to determine the specific rotation, [α], of sucrose. Give this value to two decimal places. State the co-ordinates of both points you used in your calculation. co-ordinates 1 .............................................. co-ordinates 2 .............................................. specific rotation of sucrose, [α] = ..............................................[2]
4. (b)(i)You are asked to write instructions for another student to follow so they can prepare a standard solution of 250 cm³ 0.0750 g cm⁻³ sucrose. The student is provided with solid sucrose and a 250 cm³ volumetric flask. Calculate the mass, in g, of sucrose the student would need to use. mass of sucrose = .............................................. g[1]
5. (b)(ii)Describe how the student should accurately prepare the standard solution using a sample of sucrose of mass calculated in (i).[2]
6. (c)(i)The student used the standard solution prepared in (b) to prepare the solutions in the table on page 8. Calculate the volume of standard solution of concentration 0.0750 g cm⁻³ and the volume of distilled water needed to prepare 15.00 cm³ of sucrose solution of concentration 0.0350 g cm⁻³. Give your answers to two decimal places. volume of standard solution = .............................................. cm³ volume of distilled water = .............................................. cm³[1]
7. (c)(ii)The volumes of the two solutions given in (c)(i) could be measured using the same type of apparatus. Name a suitable piece of apparatus which could be used to measure these volumes.[1]
8. (c)(iii)In (a)(ii) you circled an anomalous point. This was caused by the student incorrectly making one of the sucrose solutions. Suggest the error made by the student that caused this anomaly.[1]
9. (d)The student recorded the observed angle of rotation, αobs, for a sucrose solution of unknown concentration as +3.75. Determine the concentration of this sucrose solution in mol dm⁻³. [Mᵣ, sucrose: 342] concentration of sucrose = .............................................. mol dm⁻³[3]
10. (e)The glass cell of 10 cm length is expensive, so one cell is used for all the solutions that are placed in the polarimeter. Suggest how you would ensure that the concentration of solution in the cell is accurate each time the cell is used for the different sucrose solutions.[1]
11. (f)Concentration of sucrose is the independent variable in this polarimeter experiment. The glass cell of 10 cm length is replaced by a glass cell of 20 cm length. The 20 cm glass cell is filled with 0.0750 g cm⁻³ sucrose solution. Predict the value for the observed angle of rotation, αobs, for the sucrose solution of concentration 0.0750 g cm⁻³ when the 20 cm cell is used. Explain your answer. predicted value = .............................................. explanation ....................................................................................................................................[2]
12. (g)Before the angles of rotation of the sucrose solutions are measured, the glass cell is first filled with distilled water and the angle of rotation measured. Explain why this measurement is taken.[1]

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